Solid Freeform Fabrication (“SFF”) is a set of manufacturing processes that produce complex solid objects directly from geometric models without specific part or tooling information. SFF builds up 3D objects by successive 2D layer deposition. Objects are sliced into 2D thin layers and each layer is built by various deposition or forming processes. Deposition methods may include solidification of liquid resins with ultraviolet radiation, such as stereolithography, sintering of powders with laser scans, such as selective laser sintering, and extrusion of heated thermoplastic polymers, such as fused deposition modeling. Subsequent layers are then deposited and bonded onto the previous layers until the final approximated 2½ D objects are constructed.
These additive SFF techniques exhibit some advantages over traditional manufacturing methods. First, complex objects can be easily built; 3D objects are decomposed into 2D layers, and planning in the 2D domain is relatively simple. Second, no custom fixtures are required; traditional machining operations demand sophisticated fixturing techniques for complex objects. Third, build time is greatly reduced; parts are often completed within a few hours on up to a couple of days, in contrast to conventional processes that may take few weeks, months or longer.
However, several disadvantages can also be observed due to the nature of layered processing. First, the surface finish of completed parts is poor; parts exhibit well-known stair-step effects along the build direction. In addition, the shrinkage factor for the chosen material has to be considered during planning. Second, material choice is limited. Finally, material integrity is hard to achieve for engineering applications; constraints on material selection and geometric preservation prohibit parts built with common engineering materials. In addition, the intrinsic behavior of such fabrication methods makes materials anisotropic along the build direction. Applications of additive SFF processes are often limited to look-and-feel prototypes, form-fitting for assembly tests, or as an intermediate means for other manufacturing processes, such as patterns for investment casting. Metal parts with good surface quality, accurate dimensions, and high structural strength cannot be produced directly with the above named rapid prototyping techniques.